The effects of watershed characteristics on storm runoff relationships in Vietnam

This paper presents results of a study of watershed factors on storm runoff and daily variation of stream flow. Fifteen watersheds representing different in ecological regions, climate regimes, and forest types in Vietnam are selected for this study. The basic hydrological data set corresponding to each watershed included rainfall and stream flow recorded hourly at the watershed outlet in 2005. There are a total 830 storm events in excess of 5 mm used to analyze the relationship between factors. Peak discharge is influenced most by initial flow (m3s-1) and rainfall (mm), whereas intensity (mm hr-1) is not significant at any watershed. The lag time to peak flow (hrs) is not significant related to any watershed factors. Forest cover (%) is indirectly significant with flow coefficient of variation (%), index of increasing and decreasing flow rates (m3s-1), respectively. Forest distribution (%) is directly significant with two flow rate indices. These two independent forest variables are associated with approximately 20 -30% of total variation in responding runoff variables. Watershed size (km2) is not significantly related to any runoff indices, while shape index is directly significant with increasing and decreasing flow rate. Watershed shape explains about 27% of the total variation in the stream flow variation and increasing flow rate, respectively. Average slope (%) is not significant with any runoff variables at level 0.1. However, average elevation difference (m) is highly significant related to index of increasing and decreasing flow rates (m3s-1).